The Sun Sneeze Gene

I am a sun sneezer, which is also known as having the photic sneeze reflex, or the autosomal dominant compelling helio-ophthalmic outbursts syndrome. Which basically means if I go from a dark area into somewhere that's brightly lit - you know, like, looking at the sun - I will sneeze. Wait for it... There you go, I'm a sun sneezer.

But why does this happen? People have known about this effect for at least a few thousand years. Aristotle was probably a sun sneezer because in his book of problems he asked: Why does the sun provoke sneezing? And his answer was it was the heat from the sun which causes sweating inside your nose, and so you sneeze to get rid of the moisture. But a couple millennia later, Sir Francis Bacon demonstrated that this could not be the case because when he closed his eyes and turned towards the sun, well he did not experience that photic sneeze reflex. So his explanation was that it was the eyes watering, and then when some of that moisture got down into the nose, that caused the tickle which makes you sneeze.

But even this theory has its problems because eye watering is a much slower process than the photic sneeze reflex, so it can't be the cause. Before I started researching sun sneezing, I thought that having the photic sneeze reflex must convey some sort of evolutionary advantage under the people who have it. If you think about it, sneezing is a way of transmitting disease. In your snot, there can be the living bacteria that cause tuberculosis, and strep throat, or viruses that cause measles, mumps, rubella, and influenza. So if you're living in a moist dark cave and you sneeze, where your snot lands on the floor or the walls of the cave, those pathogens can stay alive for hours or even days, increasing the likelihood of spreading that disease to other people who live with you in your cave.

Whereas if you only sneeze when you emerge from the darkness of the cave and into bright sunlight, well, then that mucus will quickly dry out or, because it's exposed to the harmful UV rays of the sun, it will kill all of those pathogens, greatly decreasing the risk of spread of disease. By the 1960s, some studies were revealing the hereditary nature of the photic sneeze reflex. There was a father who would sneeze two times when he entered bright sunlight. And when they tested his baby daughter, who was just four weeks old, moving her from a dark room into full sunlight also caused her to sneeze exactly two times.

By the 1980s, it was clear that this trait was autosomal dominant, meaning that you only needed to inherit one copy of the gene from just one of your parents in order to exhibit the trait. But when studies of the population have been done, only about 18 to 35 percent of people actually have the photic sneeze reflex. So I don't really think this gives a great evolutionary advantage; otherwise everyone would have it. You know, it could just be one of those random mutations that happens over time and sustains itself because it's neither good nor bad evolutionarily speaking, so it affects roughly one in four people. Can you spot the person with the photic sneeze reflex?

Nowadays, it's actually been found that the gene responsible for the photic sneeze reflex is on the second chromosome, and it's a single letter of DNA that's been changed. On my second chromosome, I have a C where non-sun sneezers have a T. And the reason that we know this is that back in 2010, there was a study of about 10,000 people where they went online and they reported whether they were sun sneezers or not, and then their DNA was analyzed and the groups were compared.

What they found was the thing the sun sneezers had in common was this one particular letter change in their DNA. That I think is pretty amazing. Now it's unclear exactly how this change affects your physiology and makes you more susceptible to sun sneezes, but the best theory at the moment is that it involves the trigeminal nerve, which is the largest cranial nerve. It involves all of the feeling that you have in your face and, as the name implies, it actually has three branches, one of which receives stimuli from your eye and another which receives stimuli from your nose.

So the thought is that this really active stimulation of the optic nerve may cross over into the maxillary branch, causing that little tickle which gets you to sneeze. Photic sneezes are generally pretty harmless unless you're doing something like flying a fighter jet or performing some sort of delicate surgery. But this methodology can allow us to learn a lot about different heritable traits and diseases just by studying lots of people and their genotypes.

This episode of Veritasium was supported by 23andMe, which is actually the company that figured out that gene polymorphism responsible for the photic sneeze reflex. The name of the company, 23andMe, comes from the fact that humans actually have 23 pairs of chromosomes. And the purpose of the company is to give people insight into their own DNA.

So what you do is spit into a vial, which is actually much harder than it sounds, and send it off to them. And of course in your saliva are some of the cells from your mouth. They analyze the DNA from those cells, and then they send you a report where you can find out tons of stuff about your genes, like where do your genes come from, that is what's your ancestry, and do you have the photic sneeze reflex? Do we share the same change of DNA on our second chromosome?

You can also learn about your health and other physical traits and how much Neanderthal you are. So if you want to learn more about your genotype, then you should go to 23andme.com/veritasium. So I want to thank 23andMe for supporting me. And, I want to thank you for watching.